These days, blogs and news reports about global warming often include the optimistic report that emissions worldwide, and in the U.S., are on the decline. Yes, that’s a good thing. It’s important, it’s crucial.
But one wonders, how well are we doing? I’m not speaking in terms of emissions, but of the buildup of atmospheric CO2. For that, we should not look at emissions data but at concentration data, which (if I’m not mistaken) are more accurate and more reliable than emissions data.
Here’s the concentration (in “ppmv,” parts per million by volume) since 1958 measured at the atmospheric observatory at Mauna Loa in Hawaii:
It’s still increasing, which is no suprise because emissions haven’t dropped to zero. But according to reports they have dropped, so we might hope CO2 concentration isn’t rising as fast as it was (i.e., that CO2 is decelerating). Let’s find out.
The first thing I’ll do is remove the annual cycle. CO2 peaks in May because land plants (mostly in the northern hemisphere) have decayed over the winter, returning to the air the CO2 they captured while growing. It bottoms out in October after a summer season of growth by extracting CO2 from the atmosphere for the carbon.
When we subtract the annual cycle (I did so by a 4th-order Fourier fit) we’re left with this:
Now we’re ready to estimate the rate at which CO2 is increasing.
There are many ways to do so. One is to compute annual averages and take the differences from year to year (I computed annual averages April-through-March so as to include the most recent data, and compensated for those few months with missing data). Another is to fit a smooth curve which can also estimate the rate of increase. I did both, and here are the results (year-on-year in black, by smooth fit in red):
Back in 1960, CO2 was only increasing at about 0.77 ppmv/yr. Now it’s rising about 2.4 ppmv/yr.
More to the point, there’s no sign of any decrease in the rate (deceleration of CO2). In fact the rate seems to be increasing still (acceleration of CO2). We can look more closely at the data since the year 2000, fitting a 4th-order Fourier series and a rising trend line simultaneously, then examining the residuals; a flat line would indicate a constant growth rate:
There’s a distinct visual impression that recently (since about 2012) it’s been rising faster than before 2012, and it’s been considerably higher for the last 5 months.
Part of that is almost surely due to the recent el Niño, which is known to influence CO2 concentration, probably (if I’m not mistaken, but I’m not sufficiently expert to know for sure) through its influence on plant growth and decay rates. Comparing since-2000 CO2 to MEI (the Multivariate El Niño Index), I find the strongest influence when the el Niño effect is lagged by 6 months; it models the residuals shown above thus:
What’s left over after removing the estimated el Niño influence is this:
There’s still the possibility of faster rise since about 2010, but I don’t have a lot of confidence in that conclusion. I do, however, have a lot of confidence in the conclusion that the rate of CO2 growth has not decreased. There’s been no deceleration. Whatever emissions reductions have happened, haven’t yet slowed down the rise of CO2.
I’m certainly not an expert in the carbon cycle, but there are some troubling aspects to watch out for. One is the melting of permafrost, which can release massive amounts of CO2 into the air even if we bring our emissions to a halt. Another is the possible saturation of the land-plant carbon sink. And, I have no idea how deforestation has trended, or the impact of increased wildfire on the carbon load in the atmosphere. All these factors — about which I know next to nothing — may impact CO2 in ways which make it much harder to keep the atmospheric load down.
All of which emphasizes the importance of reducing our own emissions, quickly and as much as possible. It’s past time to tell our elected officials that we won’t put up with more delays, or more catering to the fossil fuel industry. We really need to let politicians know that if they don’t act quickly, they’ll be out of a job.
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